Dynamics of dysprosium silicide nanostructures on Si(001) and (111) surfaces

The growth and coarsening dynamics of dysprosium silicide nanostructures are observed in real-time using photoelectron emission microscopy. The annealing of a thin Dy film to temperatures in the range of 700–1050 °C results in the formation of epitaxial rectangular silicide islands and nanowires on Si(001) and triangular and hexagonal silicide islands on Si(111). During continuous annealing, individual islands are observed to coarsen via Ostwald ripening at different rates as a consequence of local variations in the size and relative location of the surrounding islands on the surface. A subsequent deposition of Dy onto the Si(001) surface at 1050 °C leads to the growth of the preexisting islands and to the formation of silicide nanowires at temperatures above where nanowire growth typically occurs. Immediately after the deposition is terminated, the nanowires begin to decay from the ends, apparently transferring atoms to the more stable rectangular islands. On Si(111), a low continuous flux of Dy at 1050 °C leads to the growth of kinked and jagged island structures, which ultimately form into nearly equilateral triangular shapes.     

Silicate formation at the interface of Pr-oxide as a high-K dielectric and Si(001) surfaces

The composition and chemical bonding of the first atoms across the interface between Si(001) and the dielectric determine the quality of dielectric gate stacks. An analysis of that hidden interface is a challenge as it requires both, high sensitivity and elemental and chemical state information. We used X-ray absorption spectroscopy in total electron yield and total fluorescence yield at the Si2p and the O1s edges to address that issue. We report on results of Pr₂O₃/Si(001) as prepared by both, epitaxial growth and metal organic chemical vapor deposition (MOCVD), and compare to the SiO₂/Si(001) system as a reference. We find evidence for the silicate formation at the interface as derived from the characteristic features at the Si2p and the O1s edges. The results are in line with model experiments in which films of increasing film thickness are deposited in situ on bare Si(001) surfaces.     

Ge growth mode modification on carbon-induced Si(001)-c(4×4) surfaces

Strong Ge morphological modifications were observed upon an ordered C-pre-covered Si(001)-c(4×4) reconstructed surface used as a template as compared to the growth on bare Si(001)-(2×1) substrates. While on bare substrates, the Ge wetting layer of the Stranski–Krastanov mode has a critical thickness of approximately 3–4 monolayers (ML), with the c-(4×4) template, island nucleation already occurs after 1 Ge ML, and growth proceeds in a Volmer–Weber mode. This suggests that the C-rich surface derm associated with the c-(4×4) reconstruction is able to strongly affect the Ge wetting.     

Directed self-assembly of Ge nanostructures on very high index, highly anisotropic Si(hkl) surfaces

Families of very high-index planes, such as those which bifurcate spontaneously to form a hill-and-valley structure composed of opposing facets, provide natural templates for the directed growth of position-controlled self-organized nanostructures with shapes determined by the facet width ratio R. For example, deposition of a few ML of Ge on Si(173 100 373), corresponding to R(113/517) = 1.7, results in a field of 40-nm-wide Ge nanowires along [72 187] with a uniform period of 60 nm.     

Ordering of Ge islands on Si(001) substrates patterned by nanoindentation

Spatial organization of Ge islands, grown by physical vapor deposition, on prepatterned Si(001) substrates has been investigated. The substrates were patterned prior to Ge deposition by nanoindentation. Characterization of Ge dots is performed by atomic force microscopy and scanning electron microscopy. The nanoindents act as trapping sites, allowing ripening of Ge islands at those locations during subsequent deposition and diffusion of Ge on the surface. The results show that island ordering is intrinsically linked to the nucleation and growth at indented sites and it strongly depends on pattern parameters.     

Optical properties of GaN epilayers grown on Si (111) and Si (001) substrates

We report the observation of bright photoluminescence (PL) emission from two types of GaN epilayers grown by molecular beam epitaxy (MBE). Wurtzite phase GaN/Si (111) epilayers are grown by gas source MBE process, whereas cubic phase GaN epilayers are grown on (001) Si covered by thin SiC film in the process of Si annealing in propane prior to the GaN growth. PL emissions are identified based on the results of detailed PL and time-resolved PL investigations. For the wurtzite phase GaN we observe an efficient up in the energy transfer from bound to free excitons. This process is explained by a large difference in the PL decay times for two types (free and bound (donor, acceptor)) of excitonic PL emissions. For cubic phase GaN we confirm recent suggestion that acceptors have smaller thermal ionization energies than those in the wurtzite phase GaN.     

Site-controlled fabrication of dimension-tunable Si nanowire arrays on patterned (001)Si substrates

In this study, we fabricated well-ordered arrays of site-controlled, vertically-aligned Si nanowires on the desired areas of pre-patterned (001)Si substrates by employing the nanosphere lithographic technique in combination with the Au-assisted selective etching process. The results of transmission electron microscopy and selected-area electron diffraction analysis show that the Si nanowires that fabricated on the patterned (001)Si substrates have a single-crystalline nature and form along the [001] direction. The length of the Si nanowires was found to increase linearly with the Au-assisted etching time. Scanning electron microscopy images clearly revealed that by adjusting the sizes of the nanosphere template and the etching temperature and time, the diameter and length of the patterned Si nanowires could be effectively tuned and accurately controlled. Furthermore, the diameters of the Si nanowires produced at various temperatures and time were found to be relatively uniform over the entire length. The combined approach presented here provides the capability to fabricate a variety of size-, length-tunable 1D Si-based nanostructures on various patterned Si-based substrates.     

Structure of Ge nanoclusters grown on Si(001) by ion beam crystallization

We have studied the temperature effect on structural transitions in the Ge/Si(001) heterosystem (strained wetting layer → hut nanoclusters → dome nanoclusters), characteristic of the ion beam crystallization process. The results demonstrate that, at a given thickness of “quasi-layers” (d _Ge = 4 monolayers), increasing the substrate temperature from 350 to 550°C increases the average size of the hut nanoclusters from 17 to 28 nm and shifts their photoluminescence peaks to the infrared spectral region (from 770 to 720 meV).     

Growth of magnetic tunnel junctions on Si(001) substrates

We present in this work the growth of magnetic tunnel junctions on Si(001) substrates using a template layer technique and the implementation of the layer-by-layer method to form the oxide barrier layer. By using a Co₂Si template layer formed by deposition of Co on Si at a temperature of ∼ 300 °C, we show that it is possible to considerably reduce the reaction between transition metals with Si substrate. We have also investigated the growth of alumina (Al₂O₃) barrier layer by an alternative layer-by-layer deposition method, which consists of successive cycles of molecular-beam deposition of an Al monolayer and oxidation under an O₂ flux at room temperature. Numerous Co(Fe)/AlO_x/NiFe tunnel junctions have been fabricated on Si(001) substrates. The oxidation kinetics, the surface morphology as well as the interface roughness and abruptness are studied by means of Auger profilometry, transmission electron microscopy and atomic force microscopy. We show that it is possible to realize a uniform and homogeneous nanometer-thick AlO_x layer with smooth and sharp interfaces. Current-voltage and Kerr effect measurements are also used to investigate the electric and magnetic properties of these junctions.     

MBE Si regrowth on carbon-induced Si(001)-c(4×4) reconstructions studied by RHEED

Si(001)-c(4×4) surfaces are obtained by exposing Si(001)-2×1 surfaces at 600°C to ethylene doses that determine C coverages in the submonolayer range. This reconstruction reveals a carbon enrichment of the topmost silicon layers. As the c(4×4) reflection high energy electron diffraction pattern can be maintained in spite of rather thick Si regrowth layers, we can conclude that this C derm is able to float at the surface during the Si capping. This segregation process is strongly dependent on the growth mode. As identified by RHEED intensity oscillations, a Si step flow growth is necessary to allow carbon to float in the first four silicon top-layers. An interplay is found between the kinetic growth conditions leading to this C-segregation and those of a self-organization process of C-rich clusters that we have observed in the course of Si_1−yC_y alloy growth obtained by codeposition of silicon and carbon.     

Regular arrays of monodisperse platinum/erbium disilicide core-shell nanowires and nanoparticles on Si(001) via a self-assembled template

We have developed a process for fabricating monodisperse noble metal/rare earth disilicide core-shell nanoparticles and nanowires in regular arrays on Si(001) with a density of 5 x 10(10) / cm2, and over areas > 1 mm2. Pt deposited via physical vapor deposition on a self-assembled rare earth disilicide nanowire template combined with reactive ion etching produces arrays of nanostructures. SEM images demonstrate the ability to select nanowires or nanoparticles as a function of Pt coverage. Statistical analysis of images of Pt nanoparticle arrays yield a mean feature size of 8 nm with a size variation of +/- 0.9 nm and interparticle spacing of approximately 15 nm.     

Barium titanate thin films deposited by electrophoresis on p-Doped Si (001) substrates

Barium titanate (BaTiO3) thin films have been prepared by electrophoretic deposition on p-doped and platinum covered silicon (Si) substrates. Their structure, nanostructure and dielectric properties were characterized. The as-deposited films were polycrystalline and composed by barium titanate nanograins with an average grain size approximately 9 nm. Annealing at high temperatures promoted grain growth, so that the samples annealed at 600 degrees C presented average grain sizes approximately 24 nm. From Raman spectroscopy measurements it was found that the tetragonal (ferroelectric) BaTiO3 phase was stabilized on the films. Also, at higher annealing temperatures, cation disorder was reduced on the films. From measurements of the temperature dependence of the dielectric permittivity the corresponding paraelectric-ferroelectric phase transition was determined. The observed transition temperature (approximately 100 degrees C) was found to be below the BaTiO3 bulk or thick film values, due to the small nanosized grains composing the films.     

Free-standing array of multi-walled carbon nanotubes on silicon (111) by a field-inducing self-assembly process

In this paper, the authors bring forward a novel method named as the field-inducing self-assembly process in which free-standing MWCNTs array can be assembled perpendicularly on silicon (111) wafer. The density of free-standing MWCNTs array is controllable by adjusting the DC voltage from 0 V to 40 V. Preferential MWCNTs array is obtained by the condensation reaction between carboxylic group of MWCNTs end and hydroxyl group of silicon electrode surface under a low DC field. And the MWCNTs array can exhibit excellent field emission property.     

Epitaxy of semipolar GaN on a Si(001) substrate with a SiC buffer layer

A new method of synthesis of semipolar gallium nitride on a silicon substrate using the technology of solid-phase epitaxy of 3C-SiC nanocrystals has been suggested. It has been demonstrated that application of buffer layers of 3C-SiC and AlN enables one to form epitaxial layers of semipolar gallium nitride with layer deviation from the polar position of the c axis of a wurtzite crystal by an angle of 48°–51° at the minimal half-width of the X-ray diffraction rocking curve (ω_θ) ～ 24′. The observed bend of a cylindrical character in the structure of GaN/AlN/3C-SiC(001) is explained by the anisotropic deformation of semipolar GaN on silicon.     

Initial stages of organometallic-vapour-phase epitaxial AIGaAs grown on (001) Si

The nucleation and growth of AIGaAs on (0 0 1) Si substrates were investigated by transmission electron microscopy (TEM). It was found that the growth temperature, growth time and V/III ratio are important parameters determining the quality of the AIGaAs nucleation layers. Decreases in the growth temperature resulted in decreases in the island size and separation, while the density increased. TEM studies also revealed that the AIGaAs islands formed under low As₄ overpressures were flatter and had higher initial densities than those grown under high As₄ overpressures. The major defects present in the AIGaAs nucleation layers were microtwins (primary and multiple twins) occurring on the inclined 1 1 1 planes of AIGaAs.     

Photovoltaic properties of GaAsP core-shell nanowires on Si(001) substrate

We report on the growth and electro-optical studies of photovoltaic properties of GaAsP nanowires. Low density GaAsP nanowires were grown by Au assisted MOVPE on Si(001) substrates using a two step procedure to form a radial p-n junction. The STEM analyses show that the nanowires have cubic structure with the alloy composition GaAs?.??P?.?? in the nanowire core and GaAs?.??P?.?? in the shell. The nanowire ensembles were processed in the form of sub-millimeter size mesas. The photovoltaic properties were characterized by optical beam induced current (OBIC) and electronic beam induced current (EBIC) maps. Both OBIC and EBIC maps show that the photovoltage is generated by the nanowires; however, a strong signal variation from wire to wire is observed. Only one out of six connected nanowires produce a measurable signal. These strong fluctuations can be tentatively explained by the variation of the resistance of the nanowire-to-substrate connection, which is highly sensitive to the quality of the Si-GaAsP interface. This study demonstrates the importance of the spatially resolved charge collection microscopy techniques for the diagnosis of failures in nanowire photovoltaic devices.     

InAs/GaAs nanostructures grown on patterned Si(001) by molecular beam epitaxy

The potential benefit from the combination of the optoelectronic and electronic functionality of III-V semiconductors with silicon technology is one of the most desired outcomes to date. Here we have systematically investigated the optical properties of InAs quantum structure embedded in GaAs grown on patterned sub-micron and nanosize holes on Si(001). III-V material tends to accumulate in the patterned sub-micron holes and a material depletion region is observed around holes when GaAs/InAs/GaAs is deposited directly on patterned Si(001). By use of a 60?nm SiO(2) layer and patterning sub-micron and nanosize holes through the oxide layer to the substrate, we demonstrate that high optical quality InAs nanostructures, both quantum dots and quantum wells, formed by a two-monolayer InAs layer embedded in GaAs can be epitaxially grown on Si(001). We also report the power-dependent and temperature-dependent photoluminescence spectra of these structures. The results show that hole diameter (sub-micron versus nanosize) has a strong effect on the structural and optical properties of GaAs/InAs/GaAs nanostructures.     

Formation of iron silicide on Si(001) studied by high resolution Rutherford backscattering spectroscopy

The initial stage of the formation of iron silicides has been investigated by high resolution Rutherford backscattering spectroscopy (HRBS). Just after Fe is deposited on Si(001) at room temperature, the deposited Fe atoms dig themselves into subsurface ejecting Si atoms into outer surface. Upon annealing at 300-400 °C, significant number of Si atoms are displaced from their lattice position near the interface between the crystalline Si and the Fe-Si mixing layer. Such displaced Si diffuses into the Fe-Si mixing layer and forms the layer with the composition of Fe:Si≈1:2. This layer is considered to be a precursive state of FeSi₂.     

Si(001)衬底上方形3 C-SiC岛的LPCVD生长

Si衬底上SiC的异质外延生长深受关注,为了了解Si衬底上的成核及长大过程,采用PLCVD方法在Si(001)衬底上生长出了方形3C-SiC岛,利用Nomarski光学显微镜和扫描电子显微镜（SEM）观察了SiC岛的形状,尺寸,密度和界面形貌,结果表明,3C－SiC岛生长所需的Si原子来自反应气源,衬底上的Si原子不发生迁移或外扩散,气相中C原子浓度决定了SiC岛的生长过程。